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1. Hydraulic traits are not robust predictors of tree species stem growth during a severe drought in a wet tropical forest

   https://doi.org/10.1111/1365-2435.14235  

   Smith‐Martin, Chris M. ; Muscarella, Robert ; Ankori‐Karlinsky, Roi ; Delzon, Sylvain ; Farrar, Samuel L. ; Salva‐Sauri, Melissa ; Thompson, Jill ; Zimmerman, Jess K. ; Uriarte, María ( December 2022 , Functional Ecology)

   Severe droughts have led to lower plant growth and high mortality in many ecosystems worldwide, including tropical forests. Drought vulnerability differs among species, but there is limited consensus on the nature and degree of this variation in tropical forest communities. Understanding species‐level vulnerability to drought requires examination of hydraulic traits since these reflect the different strategies species employ for surviving drought.

   Here, we examined hydraulic traits and growth reductions during a severe drought for 12 common woody species in a wet tropical forest community in Puerto Rico to ask: Q1. To what extent can hydraulic traits predict growth declines during drought? We expected that species with more hydraulically vulnerable xylem and narrower safety margins (SM_P50) would grow less during drought. Q2. How does species successional association relate to the levels of vulnerability to drought and hydraulic strategies? We predicted that early‐ and mid‐successional species would exhibit more acquisitive strategies, making them more susceptible to drought than shade‐tolerant species. Q3. What are the different hydraulic strategies employed by species and are there trade‐offs between drought avoidance and drought tolerance? We anticipated that species with greater water storage capacity would have leaves that lose turgor at higher xylem water potential and be less resistant to embolism forming in their xylem (P₅₀).

   We found a large range of variation in hydraulic traits across species; however, they did not closely capture the magnitude of growth declines during drought. Among larger trees (≥10 cm diameter at breast height—DBH), some tree species with high xylem embolism vulnerability (P₅₀) and risk of hydraulic failure (SM_P50) experienced substantial growth declines during drought, but this pattern was not consistent across species. We found a trade‐off among species between drought avoidance (capacitance) and drought tolerating (P₅₀) in this tropical forest community. Hydraulic strategies did not align with successional associations. Instead, some of the more drought‐vulnerable species were shade‐tolerant dominants in the community, suggesting that a drying climate could lead to shifts in long‐term forest composition and function in Puerto Rico and the Caribbean.

   Read the freePlain Language Summaryfor this article on the Journal blog.

    

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2. Stem and leaf xylem of angiosperm trees experiences minimal embolism in temperate forests during two consecutive summers with moderate drought

   https://doi.org/10.1111/plb.13384  

   Guan, X. ; Werner, J. ; Cao, K.‐F. ; Pereira, L. ; Kaack, L. ; McAdam, S. A. ; Jansen, S. ( December 2022 , Plant Biology)

   Drought events may increase the likelihood that the plant water transport system becomes interrupted by embolism. Yet our knowledge about the temporal frequency of xylem embolism in the field is frequently lacking, as it requires detailed, long‐term measurements.

   We measured xylem embolism resistance and midday xylem water potentials during the consecutive summers of 2019 and 2020 to estimate maximum levels of embolism in leaf and stem xylem of ten temperate angiosperm tree species. We also studied vessel and pit membrane characteristics based on light and electron microscopy to corroborate potential differences in embolism resistance between leaves and stems.

   Apart fromA.pseudoplatanusandQ.petraea, eight species experienced minimum xylem water potentials that were close to or below those required to initiate embolism. Water potentials corresponding to ca. 12% loss of hydraulic conductivity (PLC) could occur in six species, while considerable levels of embolism around 50% PLC were limited toB.pendulaandC.avellana. There was a general agreement in embolism resistance between stems and leaves, with leaves being equally or more resistant than stems. Also, xylem embolism resistance was significantly correlated to intervessel pit membrane thickness (T_PM) for stems, but not to vessel diameter and total intervessel pit membrane surface area of a vessel.

   Our data indicate that low amounts of embolism occur in most species during moderate summer drought, and that considerable levels of embolism are uncommon. Moreover, our experimental andT_PMdata show that leaf xylem is generally no more vulnerable than stem xylem.

    

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3. Hydraulically‐vulnerable trees survive on deep‐water access during droughts in a tropical forest

   https://doi.org/10.1111/nph.17464  

   Chitra‐Tarak, Rutuja ; Xu, Chonggang ; Aguilar, Salomón ; Anderson‐Teixeira, Kristina J. ; Chambers, Jeff ; Detto, Matteo ; Faybishenko, Boris ; Fisher, Rosie A. ; Knox, Ryan G. ; Koven, Charles D. ; et al ( July 2021 , New Phytologist)

   Deep‐water access is arguably the most effective, but under‐studied, mechanism that plants employ to survive during drought. Vulnerability to embolism and hydraulic safety margins can predict mortality risk at given levels of dehydration, but deep‐water access may delay plant dehydration. Here, we tested the role of deep‐water access in enabling survival within a diverse tropical forest community in Panama using a novel data‐model approach.

   We inversely estimated the effective rooting depth (ERD, as the average depth of water extraction), for 29 canopy species by linking diameter growth dynamics (1990–2015) to vapor pressure deficit, water potentials in the whole‐soil column, and leaf hydraulic vulnerability curves. We validated ERD estimates against existing isotopic data of potential water‐access depths.

   Across species, deeper ERD was associated with higher maximum stem hydraulic conductivity, greater vulnerability to xylem embolism, narrower safety margins, and lower mortality rates during extreme droughts over 35 years (1981–2015) among evergreen species. Species exposure to water stress declined with deeper ERD indicating that trees compensate for water stress‐related mortality risk through deep‐water access.

   The role of deep‐water access in mitigating mortality of hydraulically‐vulnerable trees has important implications for our predictive understanding of forest dynamics under current and future climates.

    

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4. A reduced role for water transport during the Cenozoic evolution of epiphytic Eupolypod ferns

   https://doi.org/10.1111/nph.18667  

   Pittermann, Jarmila ; Baer, Alex ; Campany, Courtney ; Jansen, Steven ; Holmlund, Helen ; Schuettpelz, Eric ; Mehltreter, Klaus ; Watkins, Jr, James E. ( January 2023 , New Phytologist)

   The Cretaceous–Cenozoic expansion of tropical forests created canopy space that was subsequently occupied by diverse epiphytic communities including Eupolypod ferns. Eupolypods proliferated in this more stressful niche, where lower competition enabled the adaptive radiation of thousands of species. Here, we examine whether xylem traits helped shape the Cenozoic radiation of Eupolypod ferns.

   We characterized the petiole xylem anatomy of 39 species belonging to the Eupolypod I and Eupolypod II clades occupying the epiphytic, hemiepiphytic, and terrestrial niche, and we assessed vulnerability to embolism in a subset of species.

   The transition to the canopy was associated with reduced xylem content and smaller tracheid diameters, but no differences were found in species vulnerability to embolism and pit membrane thickness. Phylogenetic analyses support selection for traits associated with reduced water transport in Eupolypod 1 species.

   We posit that in Eupolypod epiphytes, selection favored water retention via thicker leaves and lower stomatal density over higher rates of water transport. Consequently, lower leaf water loss was coupled with smaller quantities of xylem and narrower tracheid diameters. Traits associated with water conservation were evident in terrestrial Eupolypod 1 ferns and may have predisposed this clade toward radiation in the canopy.

    

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5. Hydraulic integrity of plant organs during drought stress and recovery in herbaceous and woody plant species

   https://doi.org/10.1093/jxb/erac451  

   Huber, Annika E. ; Melcher, Peter J. ; Bauerle, Taryn L. ; Lunn, ed., John ( November 2022 , Journal of Experimental Botany)

   The relationship between root, stem, and leaf hydraulic status and stomatal conductance during drought (field capacities: 100–25%) and drought recovery was studied in Helianthus annuus and five tree species (Populus×canadensis, Acer saccharum, A. saccharinum, Picea glauca, and Tsuga canadensis). Measurements of stomatal conductance (gs), organ water potential, and vessel embolism were performed and the following was observed: (i) cavitation only occurred in the petioles and not the roots or stems of tree species regardless of drought stress; (ii) in contrast, all H. annuus organs exhibited cavitation to an increasing degree from root to petiole; and (iii) all species initiated stomatal closure before cavitation events occurred or the expected turgor loss point was reached. After rewatering: (i) cavitated vessels in petioles of Acer species recovered whereas those of P. ×canadensis did not and leaves were shed; (ii) in H. annuus, cavitated xylem vessels were refilled in roots and petioles, but not in stems; and (iii) despite refilled embolisms in petioles of some species during drought recovery, gs never returned to pre-drought conditions. Conclusions are drawn with respect to the hydraulic segmentation hypothesis for above- and below-ground organs, and the timeline of embolism occurrence and repair is discussed.

    

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